Electrochemical cell devices are well known and are usually constructed to contain a multitude of individual electrochemical cells that are typically arranged in stacks. Such electrochemical cell devices typically employ metallic hardware and generally fall into one of two categories--namely, electrolysis cells, in which water or another liquid is electrolytically disassociated into its components, and fuel cells, in which a fuel (e.g., hydrogen, ammonia and hydrazine) and an oxidizer (e.g., air, oxygen) are catalytically combined, usually in order to generate electricity in the process.
As is well known, these two categories of electrochemical cell devices have many structural features in common. Therefore, in an effort to simplify the discussion hereinbelow, electrochemical cell devices, constructed and operated to perform the electrolysis of water, will be specifically described.
Such devices, which generate hydrogen and oxygen gas through the electrochemical reaction of water, at the surface of catalytic electrodes separated by an ion transporting membrane (e.g., SPE.RTM. water electrolyzers) have a structure of the type in which both sides of the ion exchange membrane are covered with electrode materials.
The metallic hardware employed in these devices, on the hydrogen side, typically include, a screen package, a fret plate(s) and a hydrogen separator sheet with the hydrogen separator sheet serving to divide the hydrogen side into a wet hydrogen section and an inerted compression compartment. All hardware components on the hydrogen side, including the hydrogen separator sheet, are typically fabricated from zirconium. Zirconium is an attractive material for use in a hydrogen atmosphere because it has superior resistance to hydrogen embrittlement. It has been found, however, that pitting of zirconium hydrogen separator sheets occurs during long life operation of these electrochemical devices.
The electrode material covering both sides of the ion exchange membrane, which are platinum group metals or oxides thereof, are typically prepared by thermal fusion of metal salts (e.g., PtCl) with sodium nitrate. The fusion step is followed by an aqueous phase electrochemical reduction at hydrogen potential in sulfuric acid and subsequent repeated water rinses to remove residual sulfur and chloride species. The resulting catalyst material is reasonably understood to be devoid or substantially devoid of any residual corrosive by-products.
The present invention is based upon the discovery that such electrode materials are not devoid or substantially devoid of residual corrosive by-products and that such by-products (e.g., chloride ions) are released from the electrode structure during cell operation. The present invention is also based upon the further discovery that such released corrosive by-products are principally responsible for the observed pitting corrosion of the zirconium hydrogen separator sheets and that such corrosion is aggravated by the presence of both fluoride ions, released as degradation products of the ion exchange membrane, and ferric ions, released from iron-based system components (e.g., plumbing).
In accordance with the above, it is an object of the present invention to provide a means for resisting pitting or corrosion of metallic hardware, such as hydrogen separator sheets, in electrochemical devices and gas generating devices, while maintaining the resistance of such metallic hardware to hydrogen embrittlement.
It is a further object to provide a means for reducing the risk of perforation or gas leakage and for further increasing the reliability of such devices.